1. Field of the Invention
This invention relates to a fluorescent composition, and more particularly to a fluorescent composition which can emit green, blue or red light under low velocity electron excitation under acceleration potential below 100 V.
2. Description of the Prior Arts
A fluorescent display apparatus has been generally used in various electric or electronic display apparatus, because it can be driven with relatively low voltage, less power consumption, and exhibit bright and very clear displays. The fluorescent display apparatus performs the display of letters or patterns by impinging electrons emitted from filamentary cathodes when energized and heated upon anodes on which to deposite phosphor layers and to which to selectively apply anode voltage. The phosphor layers deposited on the anodes of the fluorescent display apparatus generally consist of a fluorescent composition which can give forth emission of high luminance by excitation of low velocity electrons, namely low acceleration voltage.
There is heretofore known a ZnO:Zn system fluorescent composition as a low velocity electron excited fluorescent composition which is used for the fluorescent display apparatus. The ZnO:Zn system fluorescent composition can be energized by luminance threshold voltage, that is, dead voltage of as low as 1 V to 2 V, and sufficient luminance for display can be obtained with anode voltage of 10 V to 20 V. Thus, it is extremely superior as the low velocity electron excited fluorescent composition. However, the ZnO:Zn system phosphor can only emit green light under electron excitation. Thus, the luminous color emitted from the fluorescent display apparatus using the ZnO:Zn system fluorescent composition is limited to green.
On the other hand, as the expansion of the applied field of the fluorescent display apparatus, there has been a great demand for diversification of the luminous color for display. For example, when effecting a warning signal, red is preferable for increasing the warning effect rather than green. Furthermore, when displaying several kinds of informations in one or plural fluorescent display apparatuses, each of the informations can be correctly and extremely easy to recognize if the luminous color in each of the informations to be displayed is displayed in different colors. In order to meet such demand, various attempts have been made to develop a fluorescent composition which emits luminous color other than green by excitation of low speed electrons. For example, it has been proposed such low speed electron excited fluorescent compositions that are prepared by mixing various electric conductive materials with a ZnS:Ag or ZnS:Cu fluorescent composition used in the conventional cathode-ray tube which can emit various luminous color under high velocity electron excitation, or a (Zn, Cd)S:Ag or a Y.sub.2 O.sub.2 S:Eu fluorescent composition. Furthermore, it has also been proposed such fluorescent composition that is prepared by mixing SnO.sub.2 which is originally an electric conductive material with Eu.
The fluorescent composition which has been produced heretofore is energized at the luminance threshold voltage of above ten volts, and the anode voltage is required to be above several ten volts in order to obtain sufficient luminance. Thus, it is unsatisfactory as a fluorescent composition for use in low velocity electron excitation. Accordingly, in the conventional fluorescent display apparatus with which a phosphor layer made of the fluorescent composition which is excited by low velocity electrons is provided in association with the ZnO:Zn phosphor layer in order to effect luminous display of different colors at the respective regions in one display portion, the driving circuit is complicated because of the difference in the luminance threshold voltage and the operating voltage of both phosphor layers. In addition, both phosphor layers emit lights in different luminance, respectively. Furthermore, the operation life and the stability are not satisfactory.
The fluorescent composition for use in high velocity electron excitation is generally made of electric insulating material. Thus, if electric conductive materials are mixed with the fluorescent composition in large volume, the electric conductivity of the fluorescent composition is improved, and the luminance threshold voltage and the driving voltage can be decreased. However, the electric conductive materials are effective only for improving the electric conductivity of the fluorescent composition without making any contribution to the luminance. Therefore, when the electric conductive materials are mixed with the fluorescent material in a high ratio to obtain the fluorescent composition for use in low velocity electron excitation, the mixing ratio of the fluorescent material is decreased by the volume of the electric conductive material. As a result, the fluorescent composition thus obtained decreases in its luminance because of the shortage of the luminous elements, and it can not give forth emission of high luminance enough for display. Furthermore, the fluorescent composition is liable to give forth uneven emission, because non-luminous electric conductive materials are ununiformly mixed therewith. Therefore, it is unacceptable from a standpoint of the display quality.
It is known that when rare earth elements are converted into substituted atoms in a matrix of fluorescent composition, energy applied to the fluorescent composition is given to the substituted atoms due to transfer resulting from resonance, and the rare earth elements emit lights of their own by the transition of the atoms. Accordingly, if the conductive material, such as, for example, SnO.sub.2 is used as the matrix of the fluorescent composition, the fluorescent composition thus obtained can be excited by low velocity electron, and the SnO.sub.2 :Eu fluorescent composition made of SnO.sub.2 to which the rare earth element, such as, for example, Eu is added has been proposed as explained hereinabove. However, the SnO.sub.2 :Eu fluorescent composition is not satisfactory in that it exhibits saturated characteristic under low luminance, and can not give forth emission of sufficient luminance.
The fluorescent composition which can emit light under low velocity electron excitation must have electric conductivity and yet must not be electrically charged. SnO.sub.2 and TiO.sub.2 are known as the matrix of the fluorescent composition having such electric conductivity, and the SnO.sub.2 :Eu fluorescent composition explained hereinabove has been proposed as one of such fluorescent compositions. However, in the SnO.sub.2 :Eu fluorescent composition, the amount of Eu added as an activator is extremely small, because Sn is a tetrad while Eu as an activator is a triad. The luminance saturation in the SnO.sub.2 :Eu fluorescent composition explained hereinabove is considered to be the low concentration of the activator.